<PAGE>

                                                                    Exhibit 10.4












                                 FIRST AMENDMENT

                                       TO

                SECURED CONVERTIBLE DEBENTURE PURCHASE AGREEMENT

                                      AMONG

                        DIGITAL DESCRIPTOR SYSTEMS, INC.

                                       AND

                         THE INVESTORS SIGNATORY HERETO






                            Dated as of March 5, 2001


<PAGE>


         FIRST AMENDMENT TO SECURED DEBENTURE PURCHASE AGREEMENT ("First
Amendment") dated as of March 5, 2001, among Digital Descriptor Systems, Inc., a
Delaware corporation (the "Company"), and the investors signatory hereto (each
such investor is a "Purchaser" and all such investors are, collectively, the
"Purchasers").

                                   WITNESSETH:

                  WHEREAS, as of December 28, 2000, the Company and Purchasers
entered into a Secured Convertible Debenture Purchase Agreement (the "Original
Agreement"), which, among other things, provides that, subject to the terms and
conditions set forth in the Original Agreement and in accordance with ss.4(2)
under the Securities Act and Rule 506 promulgated thereunder, the Company would
issue and sell to Purchasers and the Purchasers, severally and not jointly,
would purchase the Debentures and Warrants from the Company; and

                  WHEREAS, pursuant to the terms of the Original Agreement, as
of December 28, 2000, the Company and the Purchasers delivered the following:
(A) the Company executed and delivered a (i) Debenture in the principal amount
of $100,000 to each Purchaser (each, an "Original Debenture" and collectively,
the "Original Debentures"), (ii) Warrant to purchase 200,000 shares of Common
Stock to each Purchaser, (iii) direction letter to the Company's transfer agent
(the "Original Direction Letter"); and (B) the Company and Purchasers executed
and delivered a (i) registration rights agreement (the "Original Registration
Rights Agreement"), and (ii) a security agreement (the "Original Security
Agreement"); and (C) each Purchaser paid $100,000 to the Company; and

                  WHEREAS, subject to the terms and conditions set forth in this
First Amendment and in accordance with ss.4(2) of the Securities Act and Rule
506 promulgated thereunder, the Company and Purchasers desire to (A) amend the
(i) Original Agreement to increase the aggregate purchase price for the
Debentures to $800,000 (the "New Aggregate Purchase Price") and to provide that,
subject to the amendments set forth herein, the First Additional Funding Date
will be the date hereof, (ii) Original Registration Rights Agreement to reflect
new time periods and the New Aggregate Purchase Price, (iii) Original Security
Agreement and Original Direction Letter to reflect the New Aggregate Purchase
Price; and (B) issue allonges to the Original Debentures to reflect the New
Aggregate Purchase Price.

                  NOW, THEREFORE, IN CONSIDERATION of the mutual covenants
contained in the First Amendment, and for other good and valuable consideration
the receipted adequacy of which are hereby acknowledged, the Company and
Purchasers agree as follows:

                  A. Defined Terms. All defined terms used in this First
Amendment, including the defined terms used in the preliminary clauses hereto,
which are not otherwise defined herein shall have the meanings ascribed to them
in the Original Agreement.

                  B. Amendments to Original Agreement. The Company and
Purchasers hereby agree that the Original Agreement is amended as follows:

                     1. Section 1.1(a) of the Original Agreement is hereby
amended as follows:

                                      -1-

<PAGE>

                        (i) the reference in subsection (i) to"$600,000" is
                        hereby changed to "$800,000."

                        (ii) subsection (iii) is hereby deleted in its entirety
                        and substituted therefore is the following:

                             "(iii) If each of the conditions set forth in
                             Section 1.1(b), other than the condition in
                             Sections 1.1(b)(iii) and 1.1(b)(viii), have been
                             either satisfied by the Company or waived by each
                             Purchaser, then on the date of this First Amendment
                             ("First Additional Funding Date"), as contemplated
                             in Section 1.1(b) (A) the Company will, against
                             delivery of the amounts set forth in clause (B) in
                             this paragraph, deliver to each Purchaser,
                             Debentures in the aggregate principal amount of
                             25.0% of the purchase price indicated below such
                             Purchaser's name on the signature page to this
                             Agreement (the "First Additional Debentures"), and
                             (B) each Purchaser will deliver to the Company,
                             25.0% of the purchase price indicated below such
                             Purchaser's name on the signature page to this
                             Agreement in United States Dollars in immediately
                             available funds by wire transfer to an account
                             designated in writing by the Company for such
                             purpose."

                        (iii) Subsection (iv) is hereby amended by adding "other
                        than the condition in Section 1.1 (b) (x) ," after the
                        reference to "Section 1.1 (b),".

                      2. Section 1.1(b) of the Original Agreement is hereby
amended as follows:

                        (i) Subsection (iii) is hereby amended by deleting the
                        words "the 90th day following the Closing Date" and by
                        substituting therefore the words "June 11, 2001".

                        (ii) A new subsection "(x)" shall be added as follows:

                             "(x) Additional Documents in Connection with First
                             Additional Funding Date. Concurrently with the
                             First Additional Funding Date, the parties shall
                             cause to be delivered the following: (A) the
                             Company shall deliver to each Purchaser the
                             following, all of which shall be dated as of March
                             5, 2001; (1) Debentures registered in the name of
                             such Purchaser in the aggregate principal amount of
                             25.0% of the purchase price indicated below such
                             Purchaser's name on the signature page to this
                             Agreement, (2) Warrants registered in the name of
                             such Purchaser, pursuant to which such Purchaser
                             shall have the right to acquire, for every one
                             Dollar ($1) of the principal amount of the
                             Debentures acquired by it hereunder, two shares of
                             Common Stock, upon the terms and conditions set
                             forth therein, (3) the legal opinion of Owen M.
                             Naccarato, Esq., outside counsel to the Company,
                             (4) an executed First Amendment to Registration
                             Rights Agreement, (5) an executed Transfer Agent
                             Instructions, delivered to and acknowledged in
                             writing by Company's transfer agent, (6) an
                             executed First Amendment to Security Agreement, (7)
                             executed Allonges to each of the Debentures that
                             were issued as of December 28, 2000 (8) executed
                             originals of this First Amendment, and (9) executed
                             UCC-1 financing statements; and (B) each Purchaser
                             will deliver to the Company: (1) 25.0% of the
                             purchase price indicated below such Purchaser's
                             name on the signature page to this Agreement in
                             United States dollars in immediately available
                             funds by wire transfer to an account designated in
                             writing by the Company for such purpose, and (2)
                             executed originals of this First Amendment, the
                             First Amendment to Registration Rights Agreement
                             and the First Amendment to Security Agreement.

                                      -2-
<PAGE>


                      3. Section 2.1 (a) (x) of the Original Agreement is hereby
amended to insert ", as any of such documents shall be amended from time to time
" immediately after the word "Warrants".

                      4. All references to the Debentures in the Original
Agreement shall be deemed references to the Debentures, as amended from time to
time.

                      5. The "Purchase Price for Debentures" as set forth after
the signature on the signature page to the Original Agreement of each Purchaser
is hereby changed to "$400,000" from "$300,000."

                  C. Representation of Company. To induce Purchasers to enter
into this First Amendment and to consummate the funding on the First Additional
Funding Date, the Company hereby represents and warrants to the Purchasers that
no event has occurred or failed to occur which by itself or with the giving of
notice or the passage of time, or both, would constitute a default or event of
default under the Original Agreement, the Debentures or any of the Transaction
Documents.

                  D. Fees. In connection with the preparation of this First
Amendment and the related documentation, the Company shall pay Robinson
Silverman $5,000.

                  E. No Other Changes. Except as expressly set forth in this
First Amendment, the terms and conditions of the Original Agreement remain in
full force and effect, unmodified and unchanged.


                                      -3-

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have caused this First
Amendment to be duly executed by their respective signatory as of the date first
indicated above.

                            DIGITAL DESCRIPTOR SYSTEMS, INC.


                            By:__________________________
                               Michael J. Pellegrino, Chief Financial Officer

                            PURCHASERS:

                            AJW Partners, LLC
                            By: SMS Group, LLC, Manager



                            By:__________________________
                                    Corey S. Ribotsky

                            New Millennium Capital Partners II, LLC
                            By: First Street Manager II, LLC, Manager



                            By:__________________________
                                   Glenn A. Arbeitman




                                      -4-